By saying "codes for roms," do you mean cheat codes for games? Just search google for "name of the game cheats" and substitute the "name of the game" part for whatever game you want...
well you should just search for gameshark codes so... vba emulates a gameshark so it should work that same way as one for the physical gameboy (although i dont know exactly i dont own a gameshark for my gba =p )